Coated means for connecting a chip and a card

ABSTRACT

A flex or TAB product suitable for chip carrier applications wherein the flex reliability problems caused by copper dendrite growth and lead bending during power and thermal cycling are reduced by application of special coatings to lead areas of the flex tape.

This is a division of application Ser. No. 08/329.066 filed Oct. 25,1994, now U.S. Pat. No. 5,546,655 which is a division of Ser. No.07/761,182, now U.S. Pat. No. 5,360,946, filed Sep. 17, 1991.

TECHNICAL FIELD

The present invention relates to an improved flex (or TAB) productsuitable for silicon carrier or other types of chip carrierapplications, wherein the flex reliability problems caused for exampleby Cu dendrite growth and lead bending during power and thermal cyclingare substantially reduced or eliminated. More particularly, theinvention embodies a number of coatings for use in such products anddiverse methods of making and using same.

In the first embodiment, the entire flex containing openings (orwindows) in the substrate comprising polyimide, fabricated by one of thefour methods detailed hereinafter, is manufactured in such a way thatthe coating and substrate are patterned and etched at the same time.

The second embodiment of the invention relates to a liquid or colloidalcoating that is applied by, for example, silk screening on theinner/outer lead areas on a flex tape. The properties of the liquid usedin combination with the windows in the substrate allow very precisecoating with a wide latitude for alignment.

In the third embodiment of the present invention, the flex product, usedas the interconnection between carrier and printed circuit board, iscoated with a thin layer of low stress material characterized by a lowproduct of modulus and thermal expansion coefficient. The coatingprevents lead breaks and electrical shorts between adjacent leads causedby excessive lead bending upon thermal cycling.

In a fourth embodiment of the invention, a thin layer of low stresscoating material is applied at the appropriate location on the outerlead area of a flex to seal off the area around each exposed lead with apolymer means.

PRIOR ART

According to a standard flex (or TAB) fabrication process for chipcarrier applications, the lead windows in the polyimide film substrateare created after the plating of the metal pattern. This is typicallyachieved by wet etching the polyimide substrate through a resist windowon the polyimide using a caustic etch agent (e.g., aqueous KOHsolution). Following formation of the window, a standard practice is tocoat the metal pattern with a thin layer of Au. In certain cases, the Aucoating meant to protect the metal pattern does not prevent flexreliability problems such as Cu dendrite growth during power-up.

According to the prior art, one method to solve these reliabilityproblems is to spray coat the bulk of the flex over an area between theinner and outer lead area with a high stress coating material such as afilled epoxy (e.g., Scotchcast) before bonding of the inner and outerleads to substrates (e.g., silicon carrier and printed circuit board)with matching metal pads.

One reason that the aforementioned high stress coating cannot be applieddirectly over the inner/outer lead areas is the difficulty involved inmaking a metal spray mask covering these areas with fine details, suchthat during spraying, the liquid coating material does not contaminatethe critical bonding areas of these leads.

After bonding, a final coat is then sprayed over the areas containingbonded inner and outer leads.

The flex prepared by the method noted above, however, can suffer fromthe reliability problems such as Copper (Cu) dendrite growth over theinner/outer lead areas due to incomplete coverage of the flex by thefinal coat.

Also, during flex coating prior to bonding, damage can be inflicted onthe partially supported Cu leads due, for instance, to the contactbetween flex leads and spray mask as well as other physical fixturesthat can be in contact with the flex, which as a result can greatlyreduce the yield of the flex coating step.

In addition, the partially supported leads on the flex over theinner/outer lead areas after bonding to the substrates can suffer fromsevere lead bending during power and thermal cycling, leading eventuallyto lead breaks and electrical shorts due to the high thermal stressesexerted by the high stress coating.

U.S. Pat. No. 4,209,355 discloses a composite tape product having aninsulated layer with an aperture therein and a plurality of conductingleads extending in cantilevered fashion into the aperture.

U.S. Pat. No. 4,681,654 discloses a method for making flexible filmsubstrates in a continuous tape format and describes the use ofmetallizing and etching polyimide layers to provide circuit patterns onthe polyimide layer.

U.S. Pat. No. 4,721,994 discloses a semiconductor device lead framewherein a dielectric polyimide layer is bonded to the surfaces of theinner leads.

None of these references discloses solutions to the problems associatedwith the flex coating mentioned in the prior art.

Finally demountable packages in a variety of electronic applicationsrequire that one of the connections between the chip and the base towhich it is secured, known as the motherboard, be non-permanent, toallow removal and replacement of the package. In the case of apad-on-pad (POP) module, the outer leads of the flexible connectorgenerally rest on gold pads on the card, held in place by mechanicalcompression. This situation introduces new concerns about problems atthe connection which were previously solved by some kind of permanentfixture or material. For example, the problem of metallic corrosion atthe outer lead areas of the flex has been solved in the past byencapsulating these areas after bonding with a permanent polymericcoating. Clearly, some sort of non-permanent alternative must be foundfor the encapsulating coating, since corrosion protection is also anissue in the demountable package. The primary function of a coating isto prevent the introduction of contaminants or elements which coulddamage the part or create a damaging environment. Another way to achievethis would be to surround the sensitive area with a sealant which wouldkeep out contaminants and moisture.

SUMMARY OF THE INVENTION

In accordance with the present invention, the task of protecting theflex from the problems as noted above, including the flex coating yield,Cu dendrite growth, lead breaks and electrical shorts, is achieved bythe use of one or more of the embodiments of the present invention.

Following the application of the polymer coating, there are four methodsin the first embodiment detailed below that can be used satisfactorilyto create the lead window.

The first and preferred method in this embodiment by which the leadwindows can be created in the substrate comprises developing the coatingand etching the polyimide in a polyimide etchant using the coating as apermanent photoresist.

As a second alternative, the coating and polyimide are etchedsimultaneously or sequentially using an appropriate polyimide/coatingetchant with the help of a resist. Generally in this instance, thecoating should be etchable in the polyimide etchant.

As a third option, the coating is first etched using a solvent that doesnot etch the polyimide with the help of a resist and then the polyimideis etched using a polyimide etchant prior to resist removal. In thisinstance, the coating preferably should not be etchable in the polyimideetchant.

In the fourth option the coating and the polyimide are ablated using alaser through the use of masks.

The second and third embodiments noted above involve coating the flexafter the windows have been created, but before bonding, using asuitable silk screening method with preferably a low stress, filled orunfilled silicone coating. In this instance, the mask can be designed sothat the entire flex rather than just the bulk area of the flex awayfrom the inner/outer lead areas is coated.

In addition to the properties and/or characteristics mentioned above,the polymer coating material for any of the embodiments mentioned aboveshould preferably possess the following characteristics or properties:(1) good adhesion to the polyimide-metal pattern; (2) good gap-fillingproperties; (3) relatively thin and flexible so that the flex isdrapable during joining; and (4) resistant to subsequent processing andtesting conditions.

With respect to the first method in the first embodiment cited above, apolymer is used for the coating. A particularly useful polymer coatingis a partially cyclized poly-cis isoprene such as KTFR from Kodak. Itcontains an average unsaturation of one double bond per 10 carbon atomswhich is about 50% of the double bond content of the uncyclized polymer.The double bond may be present in uncyclized C₅ H₈ units and/or in sixmembered rings. A possible structure for the repeating unit is:

The number average molecular weight of the polymer determined byosmometry is generally about 65,000±5,000. Based on this value, theweight average molecular weight is about 121,700±95,000. A gelpermeation chromotographic (GPC) study that has been published relatingto this polymer reports a number average molecular weight of 46,000 anda weight average molecular weight of 141,000. A useful sensitizercompound is 2,6-bis (p-azidobenzylidene)-4-methylcyclohexane.

For the second and third methods in the first embodiment, the polymercoatings that can conveniently be used comprise a polyimide film with asuitable adhesive (for example, Thermid from National Starch), asprayed-on polyimide properly cured on the flex before etching or apartially cured polyimide dry film laminated and cured on the flex.

With respect to the fourth method in the first embodiment relating toablating the coating and the polyimide using a laser, many polymers aresuitable such as epoxies, acrylics and/or the chemical vapor depositedparylene which is not etchable by any solvent.

The low stress silicone coatings preferred in the second and thirdembodiments noted above include addition-cured based polymers such asSC-3613 (Emerson-Cuming), or JCR-6125 (Toray), optionally containing anon-electrically conductive filler, such as TiO2, or other similarmaterials.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing a typical sequence of steps to createa coated flex containing windows.

FIG. 2 is an enlarged top view of a flex design utilized in the secondembodiment found in the present invention.

FIG. 3 is a block diagram showing a typical sequence of steps toillustrate the first method in the first embodiment pursuant to thepresent invention to create a coated flex with windows.

FIG. 4 is a block diagram showing a typical sequence of steps toillustrate the second and third methods in the first embodiment pursuantto the present invention to create a coated flex with windows.

FIG. 5 is a block diagram showing a typical sequence of steps to createpursuant to the fourth method in the first embodiment pursuant to thepresent invention to create a coated flex with windows.

FIG. 6 is a top view of the screening mask used in the second embodimentin accordance with the present invention to create a flex with coatedinner and outer lead areas.

FIG. 7 discloses a silk screening process used in the second and thirdembodiments of the present invention to coat a thin layer of low stresscoating material onto a flex.

FIG. 8 is a top view of the coated outer lead area showing the coatingon the polyimide substrate.

FIG. 9. depicts a top view of the fourth embodiment and is an overallperspective view of the relationship of chip, the card and the leads.

FIG. 10. is a cross sectional view of the demountable package describedin the fourth embodiment.

FIG. 10A. is a cross sectional magnified view of the pad-on-padconnection between the upper lead and the motherboard depicted in FIG.10, with the coating in place.

FIG. 11 is is a top view of the outer lead area with sealant in place.

FIG. 12 is a cross sectional view of the outer lead area with sealantalong 12—12 of FIG. 11.

FIG. 13 is a cross sectional view of the outer lead area with sealantalong 13—13 of FIG. 11.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

Referring to FIG. 1, the standard method to create a coated flex withwindow 1 is to apply a resist 2, pattern and develop it, and then wetetch the polyimide 3 through the resist using a caustic agent. Followingthe creation of the window 4, the metal pattern 5 secured to polyimidesubstrate 3 via a seed layer 5A used to promote adhesion is plated witha thin layer of gold 6. A protective coating 7 is then applied to thebulk of the flex away from the inner/outer lead areas. With respect tothe flex utilized in three of the embodiments of the present invention,the bulk of the flex is defined as the two bands 10, 11 shown in FIG. 2.The fourth embodiment is concerned with the outer lead area of the flex.

In the first embodiment of the present invention there are four separatemethods that can be utilized to obtain a self aligned protective coatingover gold plated copper lines during flex fabrication.

In FIG. 3, which comprises the first method of the first embodiment, thepermanent photoresist 12 appearing as the coating on the final flexproduct 11 can be a poly-cis isoprene as described above or aphotoimageable polyimide.

The steps involved in the first self-aligned method comprise firstapplying the resist 12 on both sides after plating a gold layer 14 onthe copper lead 13 which is secured to the polyimide substrate 15optionally via a seed layer 13A of e.g. Chromium. Any convenient meansto secure the lead to substrate can be used. Then the resist is exposedon both sides, developed and the polyimide 15 is etched to create window16. After the creation of the windows, the resist can be further exposedand cured for enhanced stability.

Methods two and three of the first embodiment which give rise to thefinal flex product 21 as depicted in FIG. 4, comprise applying anon-imageable coating 22 on one side, and applying and patterning aresist 23 on both sides. In accordance with the second method, both thecoating 22 and the polyimide 24 are etched using a polyimide etchant; orin accordance with the third method, the coating 22 is etched firstusing a solvent that does not etch the polyimide 24, and then thepolyimide 24 etched with the help of the resist 23. The resist 23 isthen stripped. Irrespective of the method used, window 26 is formed bythe etching.

A gold plated copper line 25 comprising copper line 13 chromium seedlayer 13A and gold plating 14 is depicted in FIG. 4. Also, a similargold plated line 35 comprising copper line 13, chromium seed layer 13Aand gold plated 14 is depicted in FIG. 5.

The fourth method for the fabrication of the coated flex 31 depicted inFIG. 5 comprises forming window 32 in the coated flex by laser ablation.The lead window 32 in the polyimide film 33 are then formed by ablatingthe coating 34 and the polyimide substrate 33 using a laser.

With respect to the application of a corrosion prevention coating to theouter/inner lead areas on a flex tape according to the second embodimentof the present invention, in some instances it has been determined thatsilk screening can easily achieve the resolution necessary for one-stepcoating and the process window is very wide due to surface tensioneffects associated with certain liquid coating materials at a desiredacceptable viscosity at the edge of the window. In a typical flex 41depicted in FIG. 2, which is the top view of transparent chip 100, adielectric material is on the top and lines are on the bottom butvisible throughout and each outer lead 42 for bonding is framed in apolyimide window 43 with very narrow polyimide strip 44 between them,and each inner lead 45 or 45A flanked by one or two adjacent innerleads.

Referring to FIGS. 6, 7 and 8, as shown in FIG. 7, the flex 41comprising polyamide substrate 3, screened with a stencil 52 (asdepicted in FIG. 6) and the coating material 53 is applied to the stripsbetween the windows 63 through an opening 54 having a dimension smallerthan the dimension of the polyimide strips between the windows depictedin FIG. 8 at 61. The coating material 53 flows to the edge 64 of thewindow 63, then forms a smooth rounded edge due to surface tensionphenomenon. The result is an even coating 62 on the strip 61 with nomaterial in the window 63 or on both the inner leads (not shown) andouter leads 65. It has been determined that due to the surface tensioneffect, the process is extremely forgiving with large tolerances on boththe stencil 52 alignment and the quantity of material dispensed.

In the third embodiment described above, the flex is coated with a thinlayer of low stress material characterized by a low product of modulusand thermal expansion coefficient. This property enables the presence ofthe flex coating to impact minimally on the lead bending that occursduring power or thermal cycling. In addition to the mechanical propertyrequirement above, this coating material must also exhibit reasonablygood adhesion to polyimide and to the gold plating on the copper linesof the flex. Good adhesion in this instance is needed to prevent copperdendrite growth which may occur in the event of coating delaminationduring the operational life of the flex or stress testing. If screening(e.g., silk screening) such as that shown in FIG. 7 is the method ofchoice to apply the coating, it is highly desirable for the uncuredcoating material to have a sufficiently long working pot life (severaldays) at room temperature to avoid contamination of the coater, etc.

Examples of the flex coatings which have proved effective includesilicone based polymers such as SC-3613 (from Emerson and Cuming) orJAR-6215 (from Toray) filled with TiO₂ particles. The addition offillers is necessitated by the need to substantially increase theviscosity of the uncured coating during screening. Other suitablecoatings include (1) un-filled silicones with sufficiently highviscosities, and (2) other families of polymers exhibiting comparableproperties.

The product which is formed as a result of the use of any of the threeembodiments described above, generally possesses copper leads having athickness of below 125 μ, preferably 40 μ, a polyimide substrate havinga thickness of between 12 μ and 125 μ preferably 50 μ, a gold thicknessof between about 0.25μ and 1.2 μ, preferably 0.7 μ, and a resist-coatingthickness generally less than 125 μ preferably less than 50 μ.

In a typical flex on a dielectric substrate, the substrate mayconveniently be formed from a polyimide material. This material has theinherent disadvantage that it can absorb up to 4% of its weight in waterusually from the moisture in the air. When conducting leads arepositioned on the dielectric substrate mentioned above and are held atdifferent potentials, the moisture serves to enhance an electrochemicalreaction wherein dissolution of one of the metal leads occursaccompanied by migration of the metal ions via the moisture medium tothe other lead. This reaction ultimately forms dendrites that short outthe leads and thus the system.

In accordance with the present invention, in a flex system where thereare adjacent leads at different potentials positioned on the surface ofa dielectric polymer substrate, the formation of dendrites is preventedby disposing means between said leads that prevent the formation ofdendrites.

A preferred means that prevents the formation of dendrites in theaforementioned system is a low modulus polymer such as silicone SC 3613as described hereinbefore.

More particularly, as noted above, when employing demountable packageflexes, sealing the outer lead area against dendritic corrosion is aconcern in that the metal pad surface must be exposed to make electricalcontact, and no permanent encapsulation after placement is possible.

The present invention provides a coating design to prevent corrosion atthe outer lead area of a pad-on-pad flex. Since the main body of theflex between the inner and outer leads remains unchanged, the currentcoating design would be used for coating that particular section of theflex tape. FIG. 9 depicts a top view of the system showing chip 100,flex 101 and motherboard (card) 102. In this view, since the demountablepackage is assembled “copper lead down”, no lines are shown as they arecovered by the polyamide. FIG. 10 depicts a cross sectional view of thesystem depicted in FIG. 9.

FIG. 10 shows the motherboard 102, the flex 101 and chip 100. Adielectric material such as kapton 103 and 103A cover copper leads 104and 104A. The copper leads 104 and 104A are connected to chip 100 atsites 105 and 105A and to the motherboard at sites 106 and 106A (whichconsists of copper pad 107 and lead-tin connection 108).

FIG. 10A provides a close-up of the connection of (gold-coated) copperouter lead 104 to contact 106 which comprises a copper pad 107 on themotherboard 102 through lead-tin connection 108. Sealant 109 (not shownin FIG. 10), forms a hermetically sealed space 110 encompassing contact106.

FIG. 11 is a top view of the fourth embodiment of the invention and therectangular outlines around the exposed metal outer lead 104. When theflex 101 is mounted and compressed, the sealant 109 rectangles areforced against the surface of the motherboard 102. Thus, a small,hermetically sealed area 110 is formed around each exposed metal lead.Implicit in this design depicted is the assumption that the flex is usedcopper-down, and that there are no holes in the polyimide substrateinside the outline of the sealant rectangle.

FIG. 12 depicts a cross sectional view of the system along axis 12—12 ofFIG. 11 showing the dielectric material such as Kapton 103, copper lead104, top connection 108, sealant 109 and motherboard 102.

FIG. 13 depicts a cross sectional view of the system along axis 13—13 ofFIG. 11 showing motherboard 102, motherboard pads 108, copper lead 104,sealant 109 and a dielectric sheet 103 such as kapton®.

In referring to the drawings, certain elements of the TAB are depictedwhich are identical; however these elements in some instances areidentified by different numbers in other drawings. For example:polyimide substrate 3 in FIGS. 8 and 11 is identical to polyimidesubstrate 15 in FIGS. 3 and polyimide subsute 102 in FIGS. 10 and 10A;copper outer lead 104 of FIG. 11 is identical to copper outer lead 42 ofFIG. 2 and copper outer lead 65 of FIG. 8; the hermetically sealedarea/polyimide window 111 of FIG. 1 is identical to hermetically seedarea/polyimide window 43 of FIG. 2 and the hermetically sealedarea/polyimide window 63 of FIG. 8; the polyimide strips 61 betweenwindows of FIG. 8 is identical to the polyamide strips 44 betweenwindows of FIG. 2 and FIG. 11.

In practical use, the sealant rectangles of polymer coating can bescreened onto the flex at the same time that the rest of the coating isapplied, so that no extra steps or new processes are required. Thesealant rectangles are fully compatible with the concept of ademountable connection, and are implemented by the same mechanicalcompression used to fix the flex. The sealant rectangles serve a doublefunction: they prevent the gross introduction of contaminants, and, ifwell adhered, prevent the migration of dissolved metal between adjacentleads. This invention can be extended to any application where anenvironmental seal is necessary and a direct coating cannot be used.

An alternative includes an encapsulating coating which could bedissolved prior to demounting, but care must be taken so that the restof the package is not contaminated. This particular alternative methodwould be less efficient in field replacement applications. Any processesof this sort in replacing a module using a solvent would have to beconducted in a more controlled manner.

It is to be understood that the methods described are not limited tocopper conductors and polyimide based layers. Other conductors such asaluminum and other base layer materials such as Upilex-S and BPDA-PDApolyimide can also be used effectively.

EXAMPLE

Using the first embodiment of the first method described above, a flexhaving copper lines of a thickness of 50 μ are adhered to a polyimidesubstrate of approximately 50 μ through a Chromium containingintermediate seed layer. The article was spray coated with KTFR, resistbaked at 10 minutes at 90° C., exposed at 100 mJ/Cm₂, and developed in aKOH based polyimide etchant.

Coated flexes were also fabricated using the low stress SC-3613 siliconematerial described in the third embodiment, as well as the screeningprocess described in the second embodiment.

These flexes (containing the KTFR and silicone coatings) were subjectedto the inner lead bending test performed between 40° and 125° C. as wellas other tests devices to determine the adhesion of the coating to theflex (pressure cooker test), and the ability of the coating to preventCu dendrite growth (temperature/humidity/bias test at 80° C./80/15V for350 hours), and no dendritic growth was observed or measuredelectrically.

Having thus described our invention, what we claim is new, and desire tosecure by Letters Patent is:
 1. An electronic system comprising ademountable package comprising a chip, a card, a flexible connectorsuitable for connecting said chip to said card, said card comprising asubstrate, said flexible connector having inner and outer metalconducting leads disposed thereon, said flexible connector having anouter contact pad associated with said outer metal conducting lead andadapted to be secured to a corresponding metal pad located on said card,each said outer contact pad and said metal pad being secured in contactwith each other by compression bonding, the improvement comprising adielectric coating that fully surrounds and is adjacent to said outermetal conducting leads of said flexible connector, said flexibleconnector being connected to said card, said dielectric coatingenveloping an exposed surface of said outer contact pad associated witheach said outer lead and protecting said outer pad against dendriticcorrosion.
 2. The electronic system defined in claim 1 wherein saidouter metal conducting leads are copper.
 3. The electronic systemdefined in claim 1 wherein said dielectric coating is a low moduluspolymer.
 4. The electronic system defined in claim 3 wherein said outermetal conducting leads are copper.
 5. The electronic system defined inclaim 4 wherein a portion of said contact pad associated with each ofsaid outer conducting leads and said metal pad located on said card arein contact with solder.
 6. The electronic system defined in claim 3wherein said metal pad located on said card is gold.
 7. The electronicsystem defined in claim 3 wherein said outer metal conducting leads arecovered by the dielectric coating over sites where said outer metalconducting leads are bonded to said card.
 8. The electronic systemdefined in claim 7 wherein the outer metal conducting lead is covered bythe said dielectric coating to prevent contaminants from entering thesaid sites where said leads are bonded to said card sites.
 9. Theelectronic system defined in claim 8 wherein said dielectric coating ispolyimide.